filemerge: make the 'local' path match the format that 'base' and 'other' use

filemerge: make the 'local' path match the format that 'base' and 'other' use

If we pass a separate '$output' arg to the merge tool, we produce four files:
local, base, other, and output.  In this situation, 'output' will be the
original filename, 'base' and 'other' are temporary files, and previously
'local' would be the backup file (so if 'output' was foo.txt, 'local' would be
foo.txt.orig).

This change makes it so that 'local' follows the same pattern as 'base' and
'other' - it will be a temporary file either in the
`experimental.mergetempdirprefix`-controlled directory with a name like
foo~local.txt, or in the normal system-wide temp dir with a name like
foo~local.RaNd0m.txt.

For the cases where the merge tool does not use an '$output' arg, 'local' is
still the destination filename, and 'base' and 'other' are unchanged.

The hope is that this is much easier for people to reason about; rather than
having a tool like Meld pop up with three panes, one of them with the filename
"foo.txt.orig", one with the filename "foo.txt", and one with
"foo~other.StuFf2.txt", we can (when the merge temp dir stuff is enabled) make
it show up as "foo~local.txt", "foo.txt" and "foo~other.txt", respectively.

This also opens the door to future customization, such as getting the
operation-provided labels and a hash prefix into the filenames (so we see
something like "foo~dest.abc123", "foo.txt", and "foo~src.d4e5f6").

Differential Revision: https://phab.mercurial-scm.org/D2889

--- a/mercurial/filemerge.py	Wed Mar 21 22:36:26 2018 -0400
+++ b/mercurial/filemerge.py	Wed Mar 21 12:36:29 2018 -0700
@@ -512,8 +512,11 @@
         return False, 1, None
     unused, unused, unused, back = files
     localpath = _workingpath(repo, fcd)
-    with _maketempfiles(repo, fco, fca) as temppaths:
-        basepath, otherpath = temppaths
+    args = _toolstr(repo.ui, tool, "args")
+
+    with _maketempfiles(repo, fco, fca, repo.wvfs.join(back.path()),
+                        "$output" in args) as temppaths:
+        basepath, otherpath, localoutputpath = temppaths
         outpath = ""
         mylabel, otherlabel = labels[:2]
         if len(labels) >= 3:
@@ -533,11 +536,10 @@
                }
         ui = repo.ui
 
-        args = _toolstr(ui, tool, "args")
         if "$output" in args:
             # read input from backup, write to original
             outpath = localpath
-            localpath = repo.wvfs.join(back.path())
+            localpath = localoutputpath
         replace = {'local': localpath, 'base': basepath, 'other': otherpath,
                    'output': outpath, 'labellocal': mylabel,
                    'labelother': otherlabel, 'labelbase': baselabel}
@@ -665,17 +667,18 @@
         return context.arbitraryfilectx(back, repo=repo)
 
 @contextlib.contextmanager
-def _maketempfiles(repo, fco, fca):
-    """Writes out `fco` and `fca` as temporary files, so an external merge
-    tool may use them.
+def _maketempfiles(repo, fco, fca, localpath, uselocalpath):
+    """Writes out `fco` and `fca` as temporary files, and (if uselocalpath)
+    copies `localpath` to another temporary file, so an external merge tool may
+    use them.
     """
     tmproot = None
     tmprootprefix = repo.ui.config('experimental', 'mergetempdirprefix')
     if tmprootprefix:
         tmproot = tempfile.mkdtemp(prefix=tmprootprefix)
 
-    def temp(prefix, ctx):
-        fullbase, ext = os.path.splitext(ctx.path())
+    def maketempfrompath(prefix, path):
+        fullbase, ext = os.path.splitext(path)
         pre = "%s~%s" % (os.path.basename(fullbase), prefix)
         if tmproot:
             name = os.path.join(tmproot, pre)
@@ -683,23 +686,42 @@
                 name += ext
             f = open(name, r"wb")
         else:
-            (fd, name) = tempfile.mkstemp(prefix=pre + '.', suffix=ext)
+            fd, name = tempfile.mkstemp(prefix=pre + '.', suffix=ext)
             f = os.fdopen(fd, r"wb")
+        return f, name
+
+    def tempfromcontext(prefix, ctx):
+        f, name = maketempfrompath(prefix, ctx.path())
         data = repo.wwritedata(ctx.path(), ctx.data())
         f.write(data)
         f.close()
         return name
 
-    b = temp("base", fca)
-    c = temp("other", fco)
+    b = tempfromcontext("base", fca)
+    c = tempfromcontext("other", fco)
+    d = localpath
+    if uselocalpath:
+        # We start off with this being the backup filename, so remove the .orig
+        # to make syntax-highlighting more likely.
+        if d.endswith('.orig'):
+            d, _ = os.path.splitext(d)
+        f, d = maketempfrompath("local", d)
+        with open(localpath, 'rb') as src:
+            f.write(src.read())
+        f.close()
+
     try:
-        yield b, c
+        yield b, c, d
     finally:
         if tmproot:
             shutil.rmtree(tmproot)
         else:
             util.unlink(b)
             util.unlink(c)
+            # if not uselocalpath, d is the 'orig'/backup file which we
+            # shouldn't delete.
+            if d and uselocalpath:
+                util.unlink(d)
 
 def _filemerge(premerge, repo, wctx, mynode, orig, fcd, fco, fca, labels=None):
     """perform a 3-way merge in the working directory

--- a/tests/test-merge-tools.t	Wed Mar 21 22:36:26 2018 -0400
+++ b/tests/test-merge-tools.t	Wed Mar 21 12:36:29 2018 -0700
@@ -1585,7 +1585,7 @@
   $ hg update -q -C 2
   $ hg merge -y -r tip --tool echo --config merge-tools.echo.args='$base $local $other $output'
   merging f and f.txt to f.txt
-  */f~base.* $TESTTMP/f.txt.orig */f~other.*.txt $TESTTMP/f.txt (glob)
+  */f~base.* */f~local.*.txt */f~other.*.txt $TESTTMP/f.txt (glob)
   0 files updated, 1 files merged, 0 files removed, 0 files unresolved
   (branch merge, don't forget to commit)
 
@@ -1600,7 +1600,7 @@
   >    --config merge-tools.echo.args='$base $local $other $output' \
   >    --config experimental.mergetempdirprefix=$TESTTMP/hgmerge.
   merging f and f.txt to f.txt
-  $TESTTMP/hgmerge.*/f~base $TESTTMP/f.txt.orig $TESTTMP/hgmerge.*/f~other.txt $TESTTMP/f.txt (glob)
+  $TESTTMP/hgmerge.*/f~base $TESTTMP/hgmerge.*/f~local.txt $TESTTMP/hgmerge.*/f~other.txt $TESTTMP/f.txt (glob)
   0 files updated, 1 files merged, 0 files removed, 0 files unresolved
   (branch merge, don't forget to commit)